Conventionally known heat pump water heaters of this type include one configured to store, in a water storage tank, water to be supplied, which water is heated by a water heat exchanger, and to supply the hot water in the water storage tank to a bathtub and a kitchen (e.g. see Patent Document 1). The refrigerant circuit of the heat pump water heater includes a compressor, an evaporator, an expansion valve, and a water heat exchanger (a gas cooler). Carbon dioxide is used as the refrigerant. The evaporator includes a plurality of heat transfer tubes and a plurality of heat transfer fins. The heat transfer tubes are spaced from one another in the radial direction thereof and are arranged vertically and longitudinally. The plurality of heat transfer fins are spaced from one another and disposed in the axial direction of the heat transfer tubes. Heat exchange is effected between the refrigerant that circulates through the heat transfer tubes and the outside air by means of the heat transfer fins.
Recently, further improvement is desired with this type of heat exchanger for an increased heat exchange rate and reduced dimensions and weight, in company with the demand for higher performance and reduced dimensions of the instruments to which the heat exchanger is applied. Thus, fin-tube heat exchangers improved in these respects are proposed (e.g. see Patent Document 2). The heat exchanger of Patent Document 2 includes a plurality of heat transfer tubes and a plurality of heat transfer fins. The heat transfer tubes are spaced from one another in the radial direction thereof and are arranged vertically and longitudinally. The heat transfer fins are spaced from one another and disposed in the axial direction of the heat transfer tubes. It is taught that an increased heat exchange rate and reduced dimensions and weight of the heat exchanger are achieved when the tube outer diameter D of the heat transfer tubes is in a range of 1 mm≦D<5 mm, the longitudinal tube row pitch L1 of the heat transfer tubes is in a range of 2.5 D<L1≦3.4 D, and the vertical tube stage pitch L2 of the heat transfer tubes is in a range of 3.0 D<L2≦3.9 D.
Patent Document 1: JP-A-2006-046877
Patent Document 2: JP-A-2005-009827